1. Field of the Invention
This invention relates to a heat-sensitive recording material. More particularly, the invention relates to a heat-sensitive material having an improved protective layer.
2. The Prior Art
Adhesive-backed, heat-sensitive labels and other thermally sensitive paper stock are widely used in printers and facsimile equipment. Such labels are often placed on goods, including meats, produce or articles of manufacture which are commonly exposed to water, solvents or oleophilic materials during storage, transit or display. Typically, a bar code and/or alphanumeric information may be formed on such labels at the point of sale by imaging the label with a thermal print head.
Known recording materials have a thermally imagable layer comprising a binder, a colorless or pale leuco dye, and an acidic substance that causes the dye to change color upon the application of heat. Labels made from such materials are commonly used in grocery stores, delicatessens, and other points of retail sale of commodities sold by weight. Increasingly, they are used on other products as well. At or prior to a sale, the retailer weighs the product, commonly on a machine which integrates a scale, register, and thermal print head, and actuates the machine to deliver a thermally imprinted label indicating the price, weight, and other information in coded and/or alphanumeric form. The label is then affixed to the product, typically by means of a pressure-sensitive adhesive backing layer.
Labels of this type are often exposed to water, fats, oils, and other solvents which can have an adverse effect on the thermal image. For example, these solvents may increase background discoloration, or, in some cases, destroy the machine readability of the imprinted bar codes. The labels are often supplied in strips to facilitate rapid printing. It has also been observed that on occasion such labels cause a discoloration to appear on red meat directly beneath the label.
The use of protective coatings on thermally sensitive materials to protect the thermal image from the deleterious effects of solvents is known. U.S. Pat. No. 4,388,362 to Iwata et al. teaches the application of a water-soluble, resinous protective coating over the heat sensitive layer. Also, U.S. Pat. No. 4,591,887 to Arbree et al. teaches the deposition of a resinous protective layer which is covalently cross-linked by melamine formaldehyde in situ to impart significantly improved solvent resistance. However, these protective layers are subject, in varying degrees, to image fading and penetration by solvents because of the character of the materials from which they are made.
U.S. Pat. No. 4,370,370 to Iwata et al. discloses thermosensitive recording adhesive label having a barrier layer comprised of a carboxylated polyvinyl alcohol which, after the polyvinyl alcohol has been coated, is treated with a metal salt solution to improve water resistance. This treatment may result in a surface cross-linking.
Despite these disclosures there remains a need in the art for thermosensitive recording adhesive labels which provide additional fade resistance and increased resistance to solvents. It is therefore an object of the present invention to provide a thermosensitive recording label capable of retaining images formed on a label for a long period of time despite long term contact with organic resin films. It is a further object of the invention to provide a thermosensitive recording label which has improved solvent resistance. Other objects of the invention will be apparent to those having ordinary skill in the art upon reading this disclosure.